1. Field of the Invention
The present invention relates to an image forming apparatus capable of correcting unevenness of an image in a main scanning direction.
2. Description of the Related Art
An image forming apparatus may provide various image qualities such as grainness, uniformity in a plane, character quality, and reproducibility (including color stability). Such image qualities provided by an electrophotography image forming apparatus may be influenced by uneven electrification caused by degradation of a charger which electrostatically charges a photosensitive drum, uneven exposure of a laser scanner, for example, configured to form an electrostatic latent image on a photosensitive drum, uneven development by a developing device which develops an electrostatic latent image or the like.
These unevennesses may cause uneven density and/or uneven color in a main scanning direction (orthogonal to a sheet conveying direction for forming an image on a sheet), which may disadvantageously deteriorate uniformity in a plane.
Japanese Patent Laid-Open No. 2004-163216 proposes a technology (main-scanning shading correction) of outputting a sheet on which a plurality of test patterns are printed in a main scanning direction and measuring color densities of the test patterns with a handy densitometer, for example, to correct an uneven density in the main scanning direction.
On the other hand, Japanese Patent Laid-Open No. 2006-58565 discloses a method of performing such main-scanning shading correction by using a color sensor internally mounted in an image forming apparatus.
Japanese Patent Laid-Open No. 2006-58565 discloses a technology of forming a band-shaped test pattern based on an equal image signal value in a main scanning direction of a sheet. Japanese Patent Laid-Open No. 2006-58565 further discloses a technology of rotating a sheet having a test pattern 90 degrees, setting it to a feeding unit, refeeding the sheet, and measuring the test pattern by using a color sensor within an image forming apparatus.
However, the disclosure in Japanese Patent Laid-Open No. 2006-58565 measures a test pattern after the sheet having the test pattern passes through a fixing unit twice since the sheet is rotated 90 degrees is refed to measure the test pattern with a color sensor. This may cause an error in measured value because the color value and color density value of the test pattern may change through the two fixing steps.
A special conveying path may be provided to prevent a test pattern from passing through a fixing unit twice, which however may increase the size of the image forming apparatus.